Fresh water is the most precious asset and has been more and more scarce. It is not possible to imagine life without the presence of water. In the industrial society, the need for an intensive production of food has increasingly accelerated the consumption of water. Associated to water consumption, there are generated great volumes of effluents. The rational use of water has become an issue of fundamental importance to the humankind survival. The industry, on its turn, is a great consumer of water. By way of illustration, according to ABIQUIM—Brazilian Chemical Industry Association (Responsible Action Report 2006), water consumption in the beer industry is of about 15 to 25 L of water/one L of beer; for gasoline manufacture from 7 to 10 L of water/one L of gasoline; polyethylene about 231 L of water/one kg of polyethylene; paper pulp: 300 to 800 L of water/one kg of paper pulp; and fine paper: 900 to 1,000 L of water/one kg of fine paper. In the sugar and alcohol industry, water consumption has been reduced, as shown by the data in table 1. However, there is still a great potential to be used.
TABLE 1Survey of capture, consumption and discharge of watersYearUses (m3/tc)1990(1)1997(2)2005(3)Capture5.65.071.83Consumption1.80.92not availableDischarge3.84.15not available(1)PERH data (State Plan for Hydric Resources) 1994/95(2)CTC Survey, 34 mills of the State of Saõ Paulo(3)UNICA/CTC Survey in 2005
As it will be demonstrated throughout this text, there is a wide potential for the sugar and alcohol industry to change the condition of water importer to water exporter and this is the main motivation of the invention proposed herein.
In order to have an idea of the production volumes involved in sugar and alcohol industry in Brazil, in the 2006/2007 harvest, according to DATAGRO (a private sugarcane consulting group in Brazil), there were processed, in 325 units in operation, 426,613,891 tons of sugar cane cultivated in an area of 5,340,000 hectares (8.8% of the agriculturable area in Brazil), yielding 17,850,646 m3 of bioethanol and 30,606,677 tons of sugar. For production of these sugar and alcohol volumes, it was necessary the capture of about 767,905,004 m3 of water and there were generated about 214,207,752 m3 of vinasse and 17,064,556 tons of filter cake.
In order to better understand how to divide the water consumption and the generation of effluents along the sugar cane processing, a brief description of the sugar and ethanol manufacturing process is presented below.
The conventional process for producing sugar, alcohol and by-products (filter cake, boiler ashes, vinasse and carbonic gas and combustion gases) comprises the steps described ahead.
The sugar cane, which is manually or mechanically harvested in the plantation site, is sent to the industry where it is cleaned (via dry or wet process) and then submitted to a preparation process in which it is chopped and defibered, conveyed to extraction where it is fed into multi-stage (usually 4 to 6) countercurrent mills, where the sugar cane receives the addition of water in the last stage (imbibition), or in diffusers, (not very common in Brazil). This initial process generates the bagasse, which is sent to be burned in boilers (of medium or high-pressure) to generate steam and electric energy. The material resulting from the bagasse burning is defined by the ash and combustion gas. The extracted mixed juice is sent to the physical-chemical treatment to produce sugar and/or alcohol, depending whether the mill is a combined mill (producing sugar and alcohol), an autonomous distillery (producing solely alcohol) or a manufacturer of sugar and molasse (end syrup).
In the combined mills, generally about 50% of the processed sugar cane is destined to sugar manufacture and 50% to alcohol production.
The juice destined to the alcohol manufacture undergoes a specific physical-chemical treatment and is sent to the fermentation vats, jointly with the exhausted final run-off syrup (mother liquor) resulting from the sugar manufacture. This mixture, called must, undergoes an alcoholic fermentation process, in agitated tanks (fermenters or vats) using yeast (Saccharomyces cerevisiae), generating a typical fermented must containing from about 6% to 11% of ethanol. As a by-product of the fermentation process, it is further generated carbonic gas, in a mass amount of 1:1 in relation to the ethanol, and the fusel oil (less than 1% in mass) which is separated in a posterior distillation step. The resulting fermented must is then submitted to centrifugation, in which the yeast is separated and recycled, and the wine containing ethanol is sent to distillation. The wine is usually brought into direct contact with the steam in distillation columns, generating two streams, an ethanol stream at the top and a vinasse stream at the bottom. Due to the utilization of vapor in direct contact with the wine, there occurs the incorporation of condensate in the vinasse, and the volume generated can be somewhat between 10 and 14 times the volume of alcohol, depending on the wine alcoholic degree. In this case, the higher the alcoholic degree the lower the volume of vinasse formed and the lower the amount of vapor consumed by liter of ethanol produced. There also exists the distillation by indirect contact, in which the generated vinasse volume is smaller, since the heating vapor is not mixed with the vinasse. In this case, the generated vinasse volume is of about 6 to 8 times the alcohol volume and there is also the generation of a vapor condensate used in the heating. For the heating and distillation of the fermented wine, it is usually used the exhausted steam or the vegetal vapor produced in the pre-evaporation of the juice.
The mixed juice destined to the sugar manufacture undergoes an operation of separating the bagacillo in cush-cush type screen (and/or rotary screens), is heated to about 40° C. and, depending on the type of sugar produced, is conveyed to sulfitation (usually in columns or hydro-ejectors) in which, by addition of sulphur dioxide resulting from the sulphur burning in the burners, has its pH reduced to about 4.0 to 4.5. The sulfitation is usually used when the sugar produced is the white crystal sugar. For the production of the raw sugar (VHP, VVHP, Demerara types), the juice is not submitted to the sulfitation process.
After sulfitation, the juice receives the addition of lime milk (or calcium saccharate), in which the pH is elevated to about 7.0 to 7.2. The limed (or dosed) juice is then heated to about 105° C. For heating the mixed juice, there are usually used vegetal vapors from the bleeding of the juice evaporation, of the first (V1), second (V2) and third stage (V3) of evaporation. The temperature of the vapor is V1>V2>V3. The temperature of these vapors ranges according to the number of juice evaporation stages. Then, the juice undergoes a vaporization process (flash balloon) for removal of dissolved gases, receiving the addition of a flocculating agent (usually a polyacrylamide polyelectrolyte) and is then submitted to the decantation in static decanters (with or without trays). This operation is also commonly known as clarification.
The clarification process generates two streams: a sludge stream, containing the impurities removed from the juice and a residual amount of sugar, and a clarified juice stream, containing most part of the sugar to be processed. The sludge, after being added with bagacillo separated in the belt of bagasse effluent from the extraction (a type of “natural filtrating means”), receives the addition of lime milk and, eventually, polyelectrolyte, and is then filtrated in vacuum rotary filters or belt press filters, thus producing the filter cake which is conveyed to the plantation site, as well as the filtrated juice, which contains most of the sugars recovered from the sludge and is reconducted to the process in the juice treatment section.
The thus obtained clarified juice is sent to the evaporation in multiple effect vacuum evaporators (usually Robert type evaporators with 4 or 5 stages), yielding, after the last evaporation stage, a concentrated juice known as syrup, with a concentration of about 65° Brix. The system operates receiving an exhausted steam in the first stage, which, in an indirect contact with the juice, generates a vegetal vapor, which is sent to the second stage, in which it heats an evaporation surface, evaporating the juice coming from the first stage and generating the vegetal vapor. This operation is repeated until the last evaporation stage in which the line of vegetal vapor is connected to a barometric condenser in direct contact with cold water (coming from a cooling system: towers, sprays, etc.). The concentrated juice effluent from this late stage is known as syrup. A common practice for thermal energy economy is to effect the bleeding of part of the vapors produced in the first (V1), in the second (V2) and in the third (V3) evaporation stages. These vapors are used in the posterior operations of evaporation and crystallization (cooking), heating of the mixed juice and distillation in the alcohol manufacture.
The syrup obtained in the evaporation is sent to the posterior concentration and crystallization step, which is carried out in vacuum calender type evaporating crystallizers in systems of two or three masses.
Generally, the conventional crystallization process in the batch system takes from 3 to 5 hours, and the crystal mass thus obtained is conveyed to horizontal crystallizers provided with a cooling jacket until reaching the ambient temperature. In this step, the juice is concentrated until the crystallization point of the sucrose and the vapor used, in an indirect contact with the syrup is the vapor V1 and/or V2. The vapor effluent from the syrup concentration is usually carried out in barometric condensers in which cold water is in direct contact with said vegetal vapor. In the evaporation and crystallization operation, it can also be used a continuous equipment, commonly employed for the masses B and C.
The crystallized mass thus produced is then submitted to a centrifugation cycle, in basket centrifuges, in which the crystals are washed upon application of water and steam and then conducted to the drying and bagging steps. The run-off syrup obtained in the centrifugation is reused in the cookings for obtaining the second sugar (sugar B or magma) and, eventually, the third sugar (sugar C or magma), which are also re-circulated in the first-sugar manufacturing process. The end syrup (molasse) originated in mass B in systems with two masses, or originated in mass C (system with three masses) is conveyed to alcohol manufacture, jointly with part of the juice separated for the production of alcohol. For centrifuging intermediate masses (B and C) continuous centrifuges are used.
It should be further emphasized that, in the mills which produce only ethanol, the steps of extraction, steam and energy generation, decantation juice treatment, distillation, alcohol dehydration are identical to those previously described regarding the combined mill. The basic difference is that the juice evaporation occurs in a single evaporation stage and the vapor generated (V1) is generally used for distillating the fermented wine. The juice pre-evaporated until about 22% to 25% of solids is fully used for the preparation of the must to be fermented.
In the case of mills which produce only sugar, it is used the same sugar manufacture process of the combined mill. The commonly produced end syrup or molasse is sold to mills which produce ethanol and to other industrial purposes as well.
For carrying out the unitary sugar cane operations previously described until obtaining the sugar and the ethanol, it should be basically eliminated almost the whole water coming from the raw material (sugar cane) and which represents about 70% of the total. This water is removed along the steps of juice evaporation, concentration and crystallization of the syrup sugar, sugar drying, ethanol distillation and dehydration.
Finally, it should be further emphasized that a practice that has been increased in the sugar and alcohol industry is the harvest of the whole sugar cane (stems and straw), that is, with part of the straw (fine straw) constituent thereof. Part of this straw remains in the field and the straw carried with the sugar cane (stems) is separated in the industry and sent to the boilers for generation of steam and, posteriorly, energy for consumption of the sugar and alcohol industry complex itself, as well as for exportation to the public electrical network. This practice has been increasingly adopted as the sugar cane burning is being gradually eliminated, with the mechanization of the crop process and the resource availability (investment) of the public sector for the construction of energy co-generation units. Thus, there is a real possibility of increasing the profitability of the agroindustrial complex, of reducing the particulate emission and substituting non-renewable energy sources.
Water is also used in the process as a fluid for: cleaning, condensation, dilution, solvent, heating, cooling, generation of vapor, extraction of sucrose from the sugar cane, control of particulate material emission, consumption and the like.
The sugar cane circuits of washing, vacuum formation and fermentation cooling are usually closed circuits. But the total hydric circuit is not closed, as the treatment of the residual waters and of the vinasse is hardly carried out and thus there are losses that make the water capture necessary. The residual waters and the vinasse are discharged on the plantation site in a fertirrigation system.
The main uses of the water and the average value are presented in Table 2 (ELIA NETO, A.: “Workshop about the charging for water use”—Convention AIAA Hydrographic Basin Committee of the Piracicaba, Capivari and Jundiai Rivers (CBD-PCJ), Piracicaba, 1996), in the case of mill with production mix of 50% of sugar and 50% of ethanol. The estimated average use of water, according to Table 2, is of 21.00 m3/sugar cane ton.
This value corresponds to much lower levels of capture, consumption and discharge of water, by reusing the water. This consumption may be summarized in three great categories: process, 29.8%; cooling, 40.2% and vacuum, 30%.
It is observed that only by eliminating the sugar cane washing it is possible to reduce about 50% of the water capture. This operation requires a high water volume and an equally high consumption, since it needs constant effluent purges with high BOD load, which requires a higher amount of make up water. These numbers indicate the perspectives for the sugar and alcohol industry as to the reduction of water consumption. Although the sugar and alcohol industry has been implementing measures for reducing water capture and waste disposal, as pointed out by ELIA NETO in his study carried out for the State of sao Paulo (ELIA NETO, A.: “Workshop about the charging for water use”—Convention AIAA Hydrographic Basin Committee of the Piracicaba, Capivari and Jundiai Rivers (CBD-PCJ), Piracicaba, 1996), it is observed that the consumption in said sugar and alcohol industry is much higher than that observed in other industries. In 2006, the chemical industry captured an average of 7.25 m3 of water/manufactured product ton (ABIQUIM—Brazilian Chemical Industry Association, 2006), whilst for production of only ethanol, considering the productivity of 84 ethanol liter/sugar cane ton, the capture of 1.83 m3/sugar cane ton (UNICA—Union of Sugar Cane Agribusiness of Sao Paulo, 2005), it would be consumed 28 m3 of water/produced ethanol ton, which indicates a consumption approximately four times higher than the average of the chemical industry.
Table 2 below presents the average values of the water uses in sugar and ethanol mills (CTC—Sugar Cane Technology Center, 1995).
AverageuseSectorUse(m3/tc)ClassificationSugar caneSugar cane washing5.33ProcessreceptionExtractionImbibition0.25Process(Mills)Bearing Cooling0.15CoolingJuice treatmentLime Milk preparation0.01ProcessCooling in the0.05CoolingsulfitationImbibition of the filters0.04ProcessCondensers of the filters0.30VacuumJuiceCondensers/multi-jets2.00VacuumconcentrationevaporationCondensers/multi-jets4.00VacuumcookersDilution of run-off0.03ProcesssyrupsCrystallizer cooling0.05CoolingSugar cane washing0.01ProcessEnergyProduction of vapor0.50ProcessgenerationCooling of the0.20CoolingturbogeneratorsFermentationJuice Cooling1.00CoolingCooling of the fermenters3.00CoolingDistilleryCooling of the condensers4.00CoolingOthersCleaning of floors and0.05ProcessequipmentPotable use0.03ProcessTotal21.00
Besides the environmental issues, high water consumption and the generation of effluents may, in a near future, impair the profitability of the business, since there are indications that, in Brazil, the capture and use of water will be charged. In 2007, the State of São Paulo started to charge the users of hydric resources relative to the volumes of capture, consumption and discharge of effluents with organic load. This charge occurs by act of State law 12183/05, ruled by State Decree no 50667/06. The charge is made decentralizedly for each of the 21 Hydrographic Basin Committees of the State of São Paulo, since they have a better knowledge of the hydric resource conditions (availability, quality, capture, etc.) of each region of the State. Nowadays, the charging is limited to R$ 0.01 (US$ 0.0050) by captured cubic meters; R$ 0.02 (US$ 0.01) by consumed cubic meter; and the triple of the sum of the two first for each cubic meter of effluent discharged back in the water bodies. There is no doubt that the attribution of an economic value to the water will naturally cause the induction to reduce the water capture and consumption and better use thereof.
Several studies demonstrate that the water contained in the sugar cane is more than sufficient for the industrial processing thereof, since, by recycling and reuse, the external water consumption can be minimized (HSIEH, W. D., H. K. SHEEN and C. H. CHEN—1995—“An approach to zero effluent in cane sugar factories”, Proceeding of the Congress of the International Society of Sugar Cane Technologists—September 1995, Cartagena). LIMA et al. emphasize in their book (LIMA, U. DE A.; AQUARONE, E.; BORZANI W. E SCHMIDELL; W. et al.—2001) that a ground sugar cane ton produces, on average, 850 liters of juice, from which 78%-86% is water, 10% to 20% is sucrose, 0.1% to 2% is reductor sugar, 0.3% to 0.5% is ash and, between 0.5 and 1.0% are nitrogenated compounds (LIMA, U. DE A.; AQUARONE, E.; BORZANI, W. E SCHMIDELL, W. et al.—2001). Industrial Biotechnology, Fermentation and Enzymatic Processes—Vol 3—Editora Edgard Blucher Ltda, Brazil. RIBEIRO (RIBEIRO, W. M. S.—1995. 3° Seminar of the Dearborn Company for the sugar and alcohol industry, 1995, Ribeirão Preto—SP, 1995) points out that the recycle and reuse of the water in the productive process can be economically feasible, as long as the implementation of these processes can provide, to the company, reduction of cost with effluent treatment and sludge disposition; reduction of cost with captured water (in case of charging); decrease in effluent monitoring level and frequency; adequate environmental management of the hydric resources and effluents, preventing penalties from inspection organisms. According to the same author, the implementation of the water reuse and recycle processes basically follows steps, such as: fully auditting the production unit, surveying data and systematizing the hydric balance; identifying the streams of higher and lower impact, mainly regarding flow rates and potential polluters; selecting alternative treatment processes for the cases in which reuse is not applicable and in which it is necessary the previous conditioning for reuse in the process; identifying the reapplication of the water, characterizing reuse or recycle; carrying out tests in plant and simulation to evaluate the technical feasibility of the alternative implementation; evaluating tests, simulating and implementing. KESSERLINGH, S. M (MINIMIZATION AND REUSE OF WATERS IN SUGAR AND ALCOHOL INDUSTRIES: CASE STUDY, Master's Degree dissertation, Escola de Engenharia de São Carlos (Engineering School), Universidade de São Paulo (Sao Paulo State University), 2002) describes in details the issue regarding high water consumption in the sugar and alcohol industries and the consequent generation of liquid effluents which cause impacts the environment. In this study, it is presented an extense bibliographic revision of the prior art in relation to the use of the water in the sugar and alcohol industry, as well as projects for the zero-effluent program.